In a sense, the entire country of England has fallen under a dark power after World War I, and Clarissa and Septimus are just two examples of this. Through Clarissa and Septimus, whom Woolf often referred to as doubles, we see the impact of British values on the souls and minds of its citizens. Septimus went off to war believing it would make him a man, and Clarissa has always abided by the strict patriarchal social standards of British culture. In this case, the dark power is a self-destructive faith in the greatness of nation and tradition at the expense of the individual. Deep stuff.
Neither Clarissa nor Septimus is wholly unable to see the beauty of life. Clarissa enjoys the flowers and Septimus likes trees. They both have spouses who love and support them. There is some hope that they enjoy everyday life, though this is certainly less true for Septimus than Clarissa.
Clarissa clearly lives with a deep sense of isolation, depression, and keen anxiety. She feels that daily living and even crossing the street can be very dangerous. Septimus' madness is much closer to the surface. He has visions of dead people, decapitated heads talking, and birds communicating cryptic messages to him in Greek. Not good signs.
Sir William Bradshaw, undoubtedly the greatest villain in the story, comes to claim Septimus. Whatever goes on in Bradshaw’s home for the mentally ill, Septimus wants nothing to do with it. Though Bradshaw at least recognizes that there’s a problem (unlike Dr Holmes), his belief in Proportion and Conversion is menacing, to say the least.
Woolf allows Septimus’ suicide to act as redemption for Clarissa. Clarissa feels a private joy that Septimus didn’t allow Bradshaw to steal his soul. She sees his death as life-affirming – a way of showing her the beauty of existence.